<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <button>显示当前系统时间</button>
    <div>某个时间</div>
    <p>123</p>
    <script>
        //1.获取元素
        var btn = document.querySelector('button');
        var div = document.querySelector('div');
        //2.注册事件
        btn.onclick = function () {
            // div.innerText = '2021-6-23';
            div.innerText = getDate();
        }

        function getDate() {
            var date = new Date();

            var year = date.getFullYear();
            var month = date.getMonth() + 1;
            var dates = date.getDate();
            var arr = ['星期日', '星期一', '星期二', '星期三', '星期四', '星期五', '星期六']
            var day = date.getDay();

            return '今天是：' + year + '年' + month + '月' + dates + '日' + arr[day];
        }

        //我们元素可以不添加事件
        var p = document.querySelector('p');
        p.innerText = getDate();
    </script>
</body>

</html>